Shoulder replacement surgeries were first performed in the 1950's in the United States to treat severe shoulder fractures. Over the years, the implants used in shoulder replacement surgeries have been improved to provide better outcomes and to expand the clinical indications for use to include shoulder arthroplasty for degenerative conditions. Modern shoulder replacement implants are generally of two designs; anatomic and reverse.
The anatomic shoulder implants are intended to restore the natural kinematics of the shoulder by replacing the humeral head and glenoid with similarly shaped prosthetic designs that recreate normal anatomy. The anatomic shoulder implant often has a spherical humeral head and a shallow concave glenoid that articulates with the spherical head. After the intact humeral head is resected, the anatomic shoulder implants have a stem configured to be securely placed down the shaft of the humerus and the spherical head is often fixed to the stem via a mechanical taper press fit. The glenoid prosthetic component, usually made from a polymer such as ultra-high molecular weight polyethylene (UHMWPE) is either cemented directly into the remaining intact glenoid or affixed to a metallic tray, which is secured to the native glenoid bone using bone screws, cement, or similar attachment methods.
The reverse shoulder is different from the anatomic shoulder implants in that the spherical surface is placed on the remaining intact glenoid and the concave articular surface is placed on the humerus. The reverse shoulder also has a stem configured to be securely placed down the shaft of the humerus. The polymer concave articular surface is fixed to the stem using a mechanical lock. The spherical head, in the reverse shoulder, is fixed to the remaining intact glenoid using a base plate.
Anatomic shoulder implants are used in patients to treat a variety of diseases that affect the shoulder joint and cause pain. A majority of these patients have osteoarthritis where the normal load bearing articular cartilage has eroded away. Reverse shoulders are generally used in patients with a weak, irreparably torn or insufficient rotator cuff. The rotator cuff is the anatomical term used to describe the group of muscles and their tendons around the shoulder joint that stabilizes the shoulder for proper motion of the joint. The reverse shoulder implants alter the kinematics of the joint and substitute for the function of the dysfunctional rotator cuff so that other muscles like the deltoid muscle can be used to lift the arm. Reverse shoulder implants may also be used in other severe cases, such as in cases of severe glenoid bone loss, where additional stability is required.
In cases where an anatomic arthroplasty has failed, it is sometimes appropriate to revise the anatomic arthroplasty to a reverse arthroplasty. To address this situation, a few shoulder arthroplasty systems have been introduced into the market that are “convertible” from an anatomic configuration to a reverse configuration. The main advantage of these convertible designs is that they obviate the removal of the existing anatomic humeral stem. Removal of the stem is technically difficult, associated with longer operative times, increased blood loss, and higher complication rates. Most designs are convertible by using an adaptor tray that allows the spherical head component of the failed anatomic arthroplasty to be exchanged with the adaptor tray which supports the concave humeral component of the reverse arthroplasty. These adaptor trays are also called “onlay” designs as the reverse poly cup is on top of the resection plane.
The adaptor trays used in these onlay designs are not always ideal because they add thickness in the joint that is potentially undesirable. This added thickness can create “over-tensioning” of the joint that over-tensions the soft tissue around the joint. Over-tensioning the joint can lead to decreased range of motion and also can cause acromial fractures, which are difficult to treat. Nonetheless, these are not the only complications that that can arise with current shoulder implants.
Another preventable complication that can occur in shoulder arthroplasty is bone loss due to stress shielding. Press-fit stem designs which achieve fixation in one region of the humerus may preferentially shield another area. The proximal metadiaphyseal and metaphyseal stress shielding are caused by stems which achieve secure fixation distally in the diaphysis. This may lead to a decrease in the physiologic loads in the proximal aspect of the humerus. Without this load, bone loss in this area can occur and potentially lead to eventual loosening of the implant. In addition, revising a failed arthroplasty that has resulted in significant proximal humeral bone loss is very difficult. There is increased fragility of the bone making fracture much more likely. Often, these fractures involve the bony attachments of the rotator cuff tendons which often compromises shoulder function. Stem designs that have a generally cylindrical shape are particularly problematic because they require a large number of sizes to address varying patient anatomy. Anatomically, the humeral head is not centered on the shaft (diaphysis) of the humerus. The stem designs that achieve fixation in the shaft must, by necessity, have multiple humeral head options with “offset” in order to recreate normal anatomy. As a result these stem designs also require a large inventory of different sizes and offsets to recreate the normal anatomy.
Current stems can also require more bone removal than is desired by the surgeon. Bone sparing designs may allow a greater amount of native bone to be preserved. For all of the above reasons, some new stem designs have the potential to be an improvement over existing stems.
Therefore, there is a need for improved shoulder arthroplasty devices and methods of use. At least some of the challenges described herein are addressed by the embodiments disclosed below.